The present invention relates to a circuit for detecting the motion or movement of a picture image, and more particularly to a motion detecting circuit suitable for motion-adapted scanning line interpolation to be made upon interlace-to-sequential (or non-interlace) conversion of scan for a television signal from an interlace scanning to a sequential (or non-interlace) scanning.
An NTSC system for a color television signal is employed at present in Japan, U.S.A., Canada, Korea and Taiwan. The NTSC signal has an interlace structure in which an image for one frame is constructed by images for two fields. Therefore, an interlace induced problem, such as line flicker, has increased conspicuously with the improvement of resolving power of a display. One approach for removing the interlace induced problem by a motion-adapted interlace/non-interlace conversion has been suggested in "IDTV and Digital Techniques", Proceedings of Symposium at the National Convention of The Institute of Television Engineers of Japan, pp. 49 to 52, August 1986. An example of the motion-adapted interlace/non-interlace conversion is shown in FIG. 2. In the figure, an input terminal 19 receives for example, a luminance signal. The inputted luminance signal is delayed by a field memory 20 by 263H (H: one horizontal scanning period) on one hand and delayed by a line memory 21 by 1H on the other hand.
FIG. 3 is a view showing the positions of horizontal scanning lines in fields in an interlace system. In FIG. 3, the abscissa represents the time and the ordinate represents the vertical position (or horizontal scanning line) on a display screen. Now assume that the inputted luminance signal is information for the n-th line. At this time, an output of the line memory 21 is information for the (n-1)th line and an output of the field memory 20 is information for the (n-263)th line. In the case where an image is a still (or stationary) picture image, a non-interlace signal is produced by signal processing in which the (n-263)th line information is interpolated at an intermediate position between the n-th line and the (n-1)th line and information for the (n-526)th line is interpolated at an intermediate position between the (n-263)th line and the (n-264)th line, as shown in FIG. 3. This signal processing is called inter-field interpolation. Namely, in the case of a still picture image, since the correlation of signals between two adjacent fields is very high, a complete image can be reproduced by interpolating a signal in the preceding field between lines in the next field. In the circuit shown in FIG. 2, a non-interlace signal is obtained in such a manner that the interlace signal on the input terminal 19 and the output signal of the field memory 20 are passed through temporal-axis compressors 27 and 28, respectively, and are thereafter outputted through a change-over circuit 29 alternately for every one horizontal scanning period.
An interlace induced problem in the case of the still picture can be removed by the interlace/non-interlace conversion mentioned above. However, in the case of a motion (or moving) picture image faster than a field period, since a deviation of image position exists between adjacent fields, the above-mentioned inter-field interpolation will result in inconveniences such as double image, thereby deteriorating the image quality. For a motion picture image portion, therefore, an interpolation signal is produced by a mean value between information of adjacent upper and lower scanning lines in the same field. This is called in-field interpolation.
In the circuit shown in FIG. 2, an adder 22 produces a mean value between information of upper and lower lines and a signal representative of the mean value is inserted between lines of the interlace signal on the input terminal 19 by the change-over circuit 29. A mixer 23 is provided for mixing an output of the field memory 20 (or an interpolation signal for still picture) and an output of the adder 22 (or an interpolation signal for motion picture). The ratio of mixture of the two interpolation signals to each other is controlled in accordance with the degree (or amount) of a motion. Namely, a large proportion of mixture is given for the output of the adder 22 in the case where the motion is large and for the output of the field memory 20 in the case where the motion is small. Such an interlace/non-interlace conversion in which an interpolation signal is produced in accordance with the motion of a picture image is called a motion-adapted interlace/non-interlace conversion. In general, the detection of the amount of motion for effecting the motion-adapted conversion is made by judging a correlation between frames. Namely, the output signal of the field memory 20 is further delayed by a field memory 24 by 262H. Thus, an output signal of the field memory 24 is information for the (n-525)th line. Accordingly, signals having therebetween a difference equal to one frame are subtracted from each other in a subtracter 25. Since the difference between the signals subtracted from each other is small in the case where those signals have a high correlation therebetween and large in the case where the correlation is low, a motion amount judgement circuit 26 makes the judgement of the presence of a motion when the absolute value of an output of the subtracter 25 is large and makes the judgement of the absence of a motion when it is small. The ratio of mixture of the input signals of the mixer 23 (or the output signal of the field memory 20 and the output signal of the adder 22) to each other is controlled in accordance with the result of judgement by the motion amount judgement circuit 25 to produce an interpolation signal.
The other examples of a motion detecting circuit in a television signal circuit have been disclosed by JP-A No. 61-70882 which was filed in Japan by Sony Corporation on Sept. 14, 1986 and laid open on Apr. 11, 1986, JP-A No. 63-56088 which was filed in Japan by Victor Company of Japan Ltd. on Aug. 26, 1986 and laid open on Mar. 10, 1988, and JP-A No. 63-90987 which was filed in Japan by Hitachi Ltd. et al on Oct. 6, 1986 and laid open on Apr. 21, 1988.
The above-mentioned techniques have one problem when a video signal representative of a finely defined image having a resolution close to a critical vertical resolution (525 TV lines) is to be reproduced. Consider the case where a signal having a resolution close to the critical vertical resolution involves a minute motion (for example, the case where an image involves a minute upward and downward motion caused by the vibration of a camera). If such a case is judged as being a motion picture and hence an in-field (inter-line) interpolation is carried out, an image for a signal portion close to the critical vertical resolution will strongly flicker, thereby deteriorating the image quality.
For example, in the case of a large-amplitude signal (525 horizontal lines for black and white) having a critical vertical resolution, as shown in FIG. 4A, the preceding field (for example, (n-263)th line, (n-262)th line, etc.) involves white information and the current field (for example, (n-1)th line, n-th line, (n+1)th line, etc.) involves black information. If an inter-field interpolation for a still picture is carried out, the signal having the critical vertical resolution in which the successive lines include alternately white information and black information, can be accurately reproduced for any field, as shown in FIG. 4B. On the other hand, if an in-field interpolation for a motion picture is carried out, the preceding field provides a signal in which all the lines involve white information as shown in FIG. 4C while the current field provides a signal in which all the lines involve black information as shown in FIG. 4D. As a result a strong flicker appears between fields. Namely, black and white appear on the image screen alternately for every one field period.
Thus, there is a problem that the interpolation processing (or in-field interpolation) for a motion picture having a resolution close to the critical resolution may cause flicker.
When a camera involves a minute vibration in the vertical direction, even a slight positional deviation between scanning lines (for example, in order of one third) produces a considerably large difference between frames, as is apparent from FIG. 5A. Accordingly, if the ratio of the in-field interpolation to the inter-field interpolation is made large under the judgement as a motion picture involving a large motion, the above-mentioned flicker between fields occurs, thereby remarkably deteriorating the image quality. Conversely, if the inter-field interpolation is applied to all cases inclusive of the above-mentioned case where the difference between frames is large, there results in practically intolerable image deterioration such as double image when a usual small-amplitude signal representative of an image having a resolution lower than the critical vertical resolution involves a motion.